1. Field of the Invention
The present invention generally relates to a signal reception device and a method of detecting a timing of signal reception in a mobile communication system, and in particular, to a signal reception device and a method of signal reception timing detection employing an OFCDM (Orthogonal Frequency and Code Division Multiplexing) transmission scheme or a multi-carrier transmission scheme.
2. Description of the Related Art
At the present time, study is being made of radio transmission schemes for the fourth generation mobile communication system aiming at higher speed and larger capacity than the third generation mobile communication system (W-CDMA). For example, a multi-carrier CDMA (Code Division Multiple Access) scheme, which involves multiplication of spread codes in a frequency axis, has tolerance for frequency-selectivity of propagation paths arising from existence of multiple paths, which causes a problem in conventional mobile communications. Due to this tolerance, it is positively studied to apply the multi-carrier CDMA scheme to radio transmission in the fourth generation mobile communication systems. For example, reference can be made to “Multi-carrier CDMA in indoor wireless radio networks,” N. Yee et al., 1993, IEEE Personal and Indoor Mobile Radio Communication (Below, referred to as “reference 1”).
The so-called OFCDM radio transmission scheme is based on the multi-carrier CDMA radio transmission scheme, in which information symbols are duplicated along a time axis and a frequency axis, and after each of the symbols is multiplied with one chip of a spread code, the thus obtained spread signals are transmitted in parallel by OFCDM symbols at different times and a number of sub-carriers having different frequencies. Because multiplication of the spread codes is performed in the time axis and frequency axis in OFCDM, and by multiplying orthogonal spread codes, codes of multiple information channels can be multiplexed.
By parallel signal transmission using a number of sub-carriers, the symbol rate is made low and the symbol length is made long. Due to this, it is possible to suppress influence of so-called “multi-path interference”, that is, the interference between signals arriving at a signal receiver at different timings after propagating through a number of different paths (multi-path propagation paths). The multi-path interference causes degradation of signal performance, and is a problem in the mobile communication environment.
In the header of each OFCDM symbol, a redundant portion referred to as “guard interval” is provided, which corresponds to repeated transmission of the latter portion of the symbol. By the guard interval, it is possible to suppress influence of inter-symbol interference arising from the aforementioned multi-path propagation.
In the multiple propagation paths, change of a propagation path is dependent on the frequency, in other words, frequency-selectivity fading occurs, and signal transmission quality changes with the frequency. In OFCDM, however, because signals are spread in the frequency axis, the signal transmission quality can be improved due to a frequency diversity effect.
On the other hand, by applying OFCDM received signals on FFT (Fast Fourier Transform), information symbols related to each sub-carrier are restored. Then, codes the same as the spread codes multiplied at the transmitting end are multiplied in the time axis and the frequency axis, and the received signals related to each sub-carrier are combined over a period of the spread codes for de-spreading.
FIGS. 25A and 25B are schematic views exemplifying the starting position of the received signals applied to FFT, also referred to as “symbol synchronization timing”.
As shown in FIG. 25A, the symbol synchronization timing of the received signals applied to FFT is ideally set at the end of the guard interval, that is, at the header (1) of an information symbol section of an OFCDM signal. In doing so, in the section (2) of a signal applied to FFT, also referred to as “FFT window section”, the incoming signals of each delay wave (here, delay waves 1 and 2) can be extracted without inter-symbol interference.
However, as shown in FIG. 25B, when the detected symbol synchronization timing deviates from the ideal position because of influence of the propagation paths, the inter-symbol interference occurs in the FFT window section, components of adjacent OFCDM signals are extracted simultaneously, and transmission performance degrades because of the inter-symbol interference. For this reason, it is important to appropriately estimate the symbol synchronization timing.
A method of detecting the symbol synchronization timing has been proposed in the related art. Similar to the OFCDM transmission, in the multi-carrier transmission of the related art involving transmission of information symbols using a number of sub-carriers having different frequencies, because the guard interval is just repeating transmission of the latter portion of the multi-carrier transmission signal, the symbol synchronization timing can be detected by using auto-correlation of the repeated portion. For example, reference can be made to “ML estimation of time and frequency offset in OFDM system”, J.- J. v. de Beek, M. Sandell, P. O. Brjesson, IEEE Trans. Signal Proc., vol. 45, no. 7, pp. 1800-1805, July 1997 (below, referred to as “reference 2”), and “Estimation of maximum likelihood symbol timing and frequency offset of multi-carrier modulated signals”, Okada, Hara, Komaki, and Morinaga, Technical report of IEICE (The Institute of Electronics Information and Communication Engineers), RCS95-118, pp 45-50, January 1996 (below, referred to as “reference 3”).
Another method is disclosed in “A fast synchronization scheme of OFDM signals for high-rate wireless LAN”, T. Onizawa et al., IEICE Transactions on Communications, vol. E82-B, no. 2, pp. 455-463, February 1999 (below, referred to as “reference 4”). Specifically, repeated signals are inserted in the header of the information signal section, and the symbol synchronization timing is detected by using auto-correlation of the received signals measured at this repeated portion.
Still another method is disclosed in “Timing synchronization in an OFDM communication system under Frequency Selective Fading”, Hira, Ishitsu, Miake, IEICE Transactions on Communications, B Vol. J84-B No. 7 pp. 1255-1264, July, 2001 (below, referred to as “reference 5”). Specifically, after calculating correlation between known pilot signals and received signals at a receiving end, among a series of the obtained correlation values, a position related to a maximum correlation value is extracted, the correlation values prior to the position are searched for, and a position related to the most forward correlation value exceeding a preset threshold is detected. The timing corresponding to this position is used as the symbol synchronization timing.
Japanese Laid Open Patent Application No. 2003-152681 (below, referred to as “reference 6”) discloses a technique capable of detecting spread codes quickly and precisely at the signal receiving end. Specifically, in a mobile communication system using the multi-carrier CDMA, on the side of the mobile station, a number of base stations are selected as the candidates of the most appropriate cell, and the correlation of each of the base stations is calculated, thereby detecting the FFT timing of the most appropriate cells and detecting scramble codes.
However, when applying the OFCDM transmission scheme to a mobile communication environment, because of the multi-path interference, it is difficult to detect the ideal symbol synchronization timing. In the technique disclosed in the aforementioned reference 2, the influence of the multi-path interference is not taken into consideration.
In the methods of the related art, including that disclosed in the reference 3, involving detecting the symbol synchronization timing by using the auto-correlation of the repeated portion, because the correlation value series is gently-sloping, the detected timing involves a large uncertainty, and when a delay wave having high electric power is incident, the synchronization timing (position) shifts backward. Especially, in a propagation path having a large delay spread, interference with the previous symbol and the next symbol occurs, hence the auto-correlation characteristic declines greatly.
In the technique disclosed in the reference 4, in order to detect the maximum output of the auto-correlation of the received signals and to reduce the influence of the multi-path interference, a timing early by a certain value is detected as the symbol synchronization timing. In this method, however, depending on the propagation path, it is necessary to optimize the value of the timing shifted forward, and it is difficult to flexibly respond to a frequently changing propagation path condition.
In the technique disclosed in the reference 5, because the correlation values prior to the position related to the maximum correlation value are searched for a correlation value exceeding a correlation value larger than 1/a of the maximum correlation value, when a delay wave having a long delay time and high electric power is present, the reception power after FFT cannot reach a maximum, and the inter-symbol interference becomes strong. In addition, because the most appropriate threshold value changes greatly depending on a transmission path model, it is necessary to optimize the threshold value, and thus it is difficult to flexibly respond to the frequently changing propagation path condition.
The technique disclosed in the reference 6 is capable of detecting the FFT timing of the most appropriate cell by selecting a number of base stations as the candidates of the most appropriate cell. However, in this technique, the FFT timing is obtained from a timing related to a maximum of detected correlation values between received signals including all the sub-carrier components before FFT and a replica of a synchronization signal, but not by combining the correlation values while requiring the correlation value after FFT to become the maximum. Therefore, it is difficult for this technique to detect the symbol synchronization timing precisely in response to the condition of the propagation path.